Various chemical substance detection apparatus have been used up until now in extremely diverse fields including the environmental fields medical treatment, food industry, and pharmaceutical industry. Those apparatuses were designed to detect the intensity of interactions between proteins in living organisms, to detect concentrations of proteins or other chemical substances in living organisms, and to detect environmental or chemical substances by utilizing fluorescent materials or fluorescent labels. Among these, the method using fluorescent materials of course utilizes the fluorescence reactions that occur through direct or indirect binding of fluorescent materials to the chemical substance for detection. This method has the disadvantage that reactions with fluorescent materials must constantly continue during detection and monitoring of a particular chemical substance. This also creates a resultant problem that chemical substances containing fluorescent materials flow downstream of the chemical substance being detected.
A chemical substance detector that measures the quantity of chemical substances without using fluorescent labels is described in, http://www.biacore.co.jp, Analytical Chemistry vol. 57, pp. 1188A (1985). In this chemical substance detector, a chemical substance (ligand), which specifically binds to the chemical substance to be detected, is fixed on a substrate in order to measure whether the chemical substance to be detected binds to the ligand. This chemical substance detector is also capable of measuring changes over time in the binding interaction between the chemical substance to be detected and the ligand. Furthermore, this detector is capable of measuring the extent (probability) of how strongly a chemical substance binds to another substance. These types of chemical substance detection methods utilize the binding between the chemical substance fixed on a substrate and the chemical substance to be detected. Among these are methods of the known art that detect changes in the refractive index in the vicinity of the substrate surface which occur when the chemical substance to be detected is bound and adsorbed to the chemical substance fixed on the substrate.
Among the above-mentioned methods for detecting changes in the refractive index in the vicinity of the substrate surface, two methods are known in the related art. One method is a technique utilizing surface plasmon resonance, and the other is a technique for detecting phase changes of light propagating through optical waveguides.
Among the chemical substance detection sensors not using fluorescent labels, measurement apparatus employing a Mach-Zehnder interferometer have the advantage of high detection sensitivity. A chemical substance detection sensor using a Mach-Zehnder interferometer, similar to the structure illustrated in FIG. 1 (top view) and FIG. 2 (sectional view), is described in, Sensors and Actuators B, 24/25 pp. 762 (1995). In this chemical substance detection sensor, light emitted from a fixed-wavelength laser light source 51 is divided into two beams by a beam splitter 52. The divided light beams then enter a slab waveguide 63 formed on a glass substrate 55. This slab waveguide is a thin film formed on the glass substrate and has a refractive index higher than that in the glass substrate and has a thickness smaller than the wavelength of the light propagating through the thin film. In the slab waveguide, the light is confined so as not to propagate in the direction of the cross section, but is not confined in parallel with the substrate surface. The light beams divided by the beam splitter 52 respectively propagate along optical paths 53 and 54 through the slab waveguide. One light beam in the optical path 53 propagates through a region 56 where ligands 66 are fixed on the surface to specifically adsorb a chemical substance 64 to be detected. The other light beam in the optical path 54 propagates through a region 57 where the substance to be detected is not adsorbed. These two light beams are then combined and redivided by an optical coupler 58 in which interference of the two light beams occurs. The intensity of each light beam after causing the interference is then measured respectively with photodetectors 60 and 61 as the outputs changing in accordance with the phase difference between the optical paths. At this point, the phase of the light passing through the optical waveguide 53 changes in proportion to the adsorbed amount of the chemical substance to be detected, causing changes in the output difference between the photodetectors 60 and 61. These changes are caused because as shown in FIG. 2, the distribution of each light propagating through the optical paths 53 and 54 spreads out of the slab waveguide thin film 67 and when the chemical substance to be detected binds to the ligands 66, the refractive index of the light propagating through the optical path 53 changes in proportion to the adsorbed amount. Moreover, the amount of change in the phase of the light passing through the region 56, which is caused by the change in the refractive index, becomes larger than the amount of change in the phase of the light passing through the region 57. Based on this principle, the concentration of the chemical substance to be detected can be measured. The adsorbed amount of a particular chemical substance can in this way be measured by using a Mach-Zehnder interferometer.
As shown in FIG. 3, the intensities of the light after being divided and then detected with the photodetectors 60 (shown as PD1 output) and 61 (shown as PD 2 output) actually change along a trigonometric function curve, and the difference between them also changes along a trigonometric function curve. By making the phase change correspond to the abscissa of this intensity change graph and by also making the phase change correspond to the adsorbed amount of a substance, the concentration of the particular chemical substance can be measured. At this time, the relation between the amount of the phase change and the adsorbed amount of the substance is determined by measuring the phase change of the substance to be detected whose concentration is known beforehand.
The following documents disclose the related art of the present invention.